1. Field of the Invention
This invention relates to fluid transfer control apparatus and, more particularly, to the use of optically-based overfill probes for detecting when fluid being transferred into a container has reached a predetermined level.
2. Description of the Related Art
In the art of fluid transfer control, particularly as it applies to the petroleum industry, one of the more common control devices is an overfill probe for monitoring the loading of fluid into a container, such as a petroleum tanker compartment. An output signal from such a probe indicates when the fluid has reached the predetermined level, and may be used as an indicator by a fluid transfer controller for discontinuing fluid flow into the container. In this way, overfilling of the container, which is particularly hazardous when dealing with flammable liquids such as gasoline, can be avoided.
One type of overfill probe which is particularly common in the petrochemical industry makes use of an optical signal which is coupled into a medium having a relatively high index of refraction, such as a glass or non-opaque plastic. This medium is specially-shaped and commonly referred to as a "prism." The prism is shaped to cause internal reflection of the optical signal when surrounded by air. The shape of the prism and the direction at which the optical signal is coupled into the prism are such that the reflection of the optical signal within the prism redirects the signal toward a photodetector. This photodetector generates an output signal which indicates that the optical signal is being detected.
A schematic illustration of this prior art probe design is shown in FIG. 1. In the plane of the optical signal path, the prism 10 has an angled cross section which allows for two internal reflections between a light source 12 and photodetector 14. When the prism 10 is surrounded by air, the optical signal (indicated by the broken line arrow in FIG. 1) is reflected at two interfaces between the prism material and the surrounding air, and redirected toward photodetector 14. The photodetector 14 generates an electrical output signal which indicates that the optical signal is being detected. This output signal is referred to as a "permit" signal, since it indicates that the fluid level permits further loading of the container.
As shown in FIG. 1, this particular prior art prism 10 uses a forty-five degree incidence angle (relative to normal) for each of the reflections of the optical signal within the prism 10. Light source 12 and photodetector 14 are oriented in the same direction along the same surface of the prism 10. When in use, the prism is part of a probe which is located within a fluid container, usually near the top of the container. When the fluid in the container rises high enough to contact a prism surface at a location where the optical signal is incident, the forty-five degree angle is no longer sufficient to provide internal reflection of the optical signal at that interface. This is because the prism/air interface becomes a prism/fluid interface, and the fluid has an index of refraction much closer to that of the prism material than does air. According to Snell's law of refraction, (well-known in the art of optical design) the forty-five degree angle of incidence of the optical signal then results in the transmission of the optical signal through the interface due to the similarity of the relative indices of refraction. As a result, the signal is no longer detected by photodetector 14, and the corresponding change in the photodetector output signal is used to discontinue loading of the container.
One of the problems encountered with optically-based fluid overfill probes is the possibility of light exiting the prism and being reflected back into the prism from an outside reflective surface. As mentioned above, the prism/fluid interface allows transmission of light from within the prism out into the fluid. The possibility exists that a reflective surface within the fluid container could be oriented in such a way as to reflect light exiting the prism back toward the prism itself. If a sufficiently strong optical signal was reflected back at a particular angle, the reflected light could re-enter the prism and overcome the detection threshold of the photodetector, thereby resulting in the output of the probe indicating that the fluid level in the container was below the overfill level, when actually the fluid was in contact with the probe prism. While this condition may not be commonplace, it can be particularly hazardous if, for example, a pump which is loading petroleum into the container uses the signal from the probe as an indication of when to discontinue loading. The "false permit" signal could thereby cause a dangerous overflow of flammable liquid.
Another problem lies in the possible buildup of liquid on the surface of the prism. Condensation, in particular, can result when the probe is in a particularly cold location, such as a container on a tanker truck operated in a particularly cold climate. Water vapor, or other liquid vapor, can condense out on the surface of the prism, and cause leakage of the optical signal through the surface of the prism, despite the fact that the prism is not in contact with fluid in the container. If the leakage is significant enough, the amount of light detected by the photodector can drop below the overfill detection threshold and, if the photodector is connected to an automatic shutoff circuit, result in premature termination of a filling operation.